along with the speed with which

[Maybo]

His execution — along with the speed with which it occurred after his sentencing — was taken as a clear signal that the government was escalating its efforts to intimidate the protesters who have mounted one of the most serious challenges to the regime’s rule in decades. (Iran Executes Man Over Nationwide Protests by Cora Engelbrecht)

Does "which" refer to "speed"?

 

[Barque]

Yes. Both his execution and the speed with which it was carried out after sentence was passed were taken as clear signals.

 

[Maybo]

Does “it” refer to “His execution”?

 

[dunchee]

Yes.

 

[Tarheel]

For a change, I would use more words. Perhaps:

The execution--along with how soon it was carried out after sentencing ....

If you carry out an execution soon after sentencing there is, of course, no chance to appeal.